The present invention relates to a stepless speed reducer versatile for machines and vehicles.
Hydraulic stepless speed reducers, commonly called a hydrostatic speed transmission (HST), are known in the art and widely used, which employs a fluid pump motor. This type of speed reducer is excellent in changing speeds, but disadvantageously, it consumes energy, and is limited to a relatively small range of variable speeds. In order to improve the known HST speed reducers, a differential gearing is used to share the power transmission with the HST mechanism. The new system is introduced in "Hydraulic Engineering" by Tomoo Ishihara (Publisher: Asakura Shobo), and "Theory and Practice of Piston Pump Motors" by Sadao Ishihara (Publisher: Corona Co.).
The known stepless speed reducer having a differential gearing includes a first mechanical transmission line for low speed between a first and second input/output shaft ends (hereinafter referred to merely as "ends", and a second mechanical transmission line for high speed between the first and a third input/output ends. A hydraulic pump motor is connected to the second input/output end at one end thereof, and to the third input/output end at the other end. The first mechanical transmission line includes a first clutch for placing its transmission end in engagement or disengagement with a rotor, and the second mechanical transmission line includes a second clutch for placing its transmission end in engagement or disengagement with the same rotor. The two clutches are alternatively turned on, thereby selectively concluding a high speed transmission line or a low speed transmission line. The stepless speed reducer having a differential gearing is operated in the following manner:
Now, suppose that the HST transmission is employed in a vehicle. The speed ratio (output rotating speed/input rotating speed) of the both transmission lines must be controlled so that the engine may not rotate at a higher speed beyond the rate of fuel supplied to the engine. When the vehicle is driven at a slightly lower speed ratio than an intermediate speed ratio at which the first and second transmission ends are rotated at an equal speed, the first clutch is selected and prepared for connection. When the speed ratio increases and reaches the intermediate speed, the first clutch is disengaged whereas the second clutch is engaged, thereby shifting the low speed mode into the high speed mode. When the high speed mode is to be shifted into the low speed mode, the second clutch is selected and prepared for connection.
In general, a hydraulic power transmission is less efficient than the mechanical power transmission; in particular, it is inefficient when it is used for the whole range of higher speeds.
In order to overcome the problem described above, a new system has been proposed, which uses the mechanical transmission line exclusively for the high speed range while the hydraulic line is kept inoperative. In this new system a maximum speed ratio is predetermined so that when the speed ratio in the high speed mode exceeds it, the hydraulic transmission line is kept inoperative, and when the speed ratio becomes lower than the maximum speed ratio, the hydraulic transmission line is put into operation. Herein, a disadvantage arises that while the vehicle is running, the speed ratio unavoidably fluctuates around the predetermined maximum speed ratio. This fluctuation of speed ratio continually turns on and off the clutches so that they are liable to quick fatigue. Another disadvantage is a shortened life of the fluid pump motor due to the act that they are constantly subjected to differential pressure remaining in the fluid circuits.
The present invention aims at eliminating the disadvantages pointed out above, and has for its object to provide an improved stepless speed reducer.
According to the present invention, there is provided a stepless speed reducer which includes the mechanical power transmission lines for high speed and low speed, the two lines being selectively concluded to effect power transmission, wherein if the working speed ratio comes near a predetermined critical speed ratio, the operation is maintained at the critical speed ratio while the pump motor for low speed is kept inoperative and no pressure difference exists between the transmission circuits, thereby transmitting power through the line for high speed.